In ironworks and coke production plants, a coke oven gas (hereinafter will be referred to as “COG” as the case may be) is evolved as a by-product gas during a coke production process in which coal is carbonized. COG is utilized as a fuel for use in ironworks, as city gas, and as a fuel for use in electric power generation systems. COG contains hydrogen and methane as major components, BTX (benzene, toluene and xylene) as volatile components of coal, sulfides, and other components.
When using COG as a fuel for, for example, a gas turbine for use in an electric power generation system, COG is compressed to a high pressure by a compressor before being supplied to the combustor of the gas turbine. When COG is in a high-pressure condition, polymerization reaction is likely to occur between nitrogen oxide (NO) and unsaturated hydrocarbon diener (including butadiene, styrene, cyclopentadiene, and indene), which are trace components of COG, to produce nitrogen oxide gum (NO gum) as a gum substance. Just after having been produced, such a gum substance is an entrained substance comprising fine particles having sizes of about 0.1 μm, which is also called “gas phase gum”. Thereafter, the gas phase gum is oxidized and polymerized to produce brown or black adhesive liquid gum.
Such liquid gum (hereinafter will be referred to as “gum substance”) adheres and deposits onto fuel gas supply piping extending to the combustor and onto any internal portion of various devices provided on the fuel gas supply piping. It is empirically known that in a gas turbine electric power generation system for example, large amounts of liquid gum are adhered and deposited onto curved internal surfaces of the fuel gas supply piping, internal surfaces of valve casings of valves, internal parts, and internal surfaces of various manifolds and like components. Deposition of a fine dust substance over the gum substance thus adhered and deposited and further deposition of the gum substance over the layers thus stacked, are supposed to occur and repeat. When the gum substance and the dust substance adheres and deposits onto the interiors of the aforementioned piping and devices, the sectional area of the flow passage of the piping is reduced, while the resistance to a movable portion of each of the devices including valves is increased. As a result, an increase in flow passage resistance inside the piping and an operation failure of any one of the valves and combustor are induced, which might forcibly lead to a sudden unexpected emergency stop of the system. As a standard method of detecting the extent of adhesion and deposition of the gum substance has not been established yet, it is impossible to predict the occurrence of an emergency stop of the system caused by deposition of the gum substance.
In cases where COG is used as city gas, when COG is pressurized in order to feed COG to a remote consumer area, the gum substance might be produced and deposited in the same manner as described above.
The followings are known as techniques of limiting the volume of gum substance production from COG or removing the gum substance produced (see patent document 1 also).
(1) NO contained in COG is removed by an iron sulfide method, activated charcoal method, high-pressure silent discharge method, pressurized retention method, or the like. These techniques, however, require large-scale systems and much driving power and, hence, the system investment cost and the running cost become high. Further, the load of maintenance work also becomes heavy. As a result, the fuel purification cost becomes high and the electric power generation cost increases substantially.
(2) COG is intentionally collected within a low-pressure gas storage tank in order to produce the gum substance. The gum substance produced and retained in COG is removed by rinsing, washing or the like. This technique, however, requires a high investment cost because of an extensive installation space and the huge gas storage tank. The running cost is also high because an enormous amount of water is needed. Further, it is not easy to improve the processing ability.
(3) Unsaturated dienes contained in COG are rendered saturated by using an expensive catalyst such as nickel or vanadium. In this technique, when the gum substance is produced in a large amount, the gum substance condenses in micropores of the catalyst to produce high polymeric gum, which rapidly lowers the performance of the catalyst. For this reason, frequent catalyst replacement is needed, which increases the running cost thereby impairing the cost efficiency of electric power generation.
Any one of the above-described processing methods is practiced at a high cost using a high-performance purification system. Even though these methods can remove much of the gum substance, they cannot completely remove the gum substance. Since COG is a gas of which properties are subject to coke production conditions, the production volume and the productions form of gum substance also change. For this reason, in some cases, the gum substance remains in COG even after the purification process.
Therefore, in examining the extent of deposition of the gum substance onto the fuel gas supply piping, the combustor or the like, it is necessary to stop the system operation and disassemble the system for checking visually the interior of the system. And further, while main processes, including the coke production process, are working, by-product gases therefrom continue to be generated. For this reason, it is impractical to stop frequently the gas turbine system and the like in order to inspect predetermined portions. Since effective prior measures cannot be taken, the system must be restored only after an emergency stop of the system has occurred.    Patent Document 1: Japanese Patent Laid-Open Publication No. SHO 59-230090